Among high-frequency and microwave resonator structures, so-called dielectric resonators have recently become increasingly interesting as they offer e.g. the following advantages over conventional resonator structures: smaller circuit sizes, higher integration level, higher efficiency and lower cost of manufacture. Any element having a simple geometric shape made of a material having low dielectric losses and a high relative dielectric constant can be used as a high Q dielectric resonator. For reasons of the manufacturing technique, the dielectric resonator is usually cylindrical, such as a cylindrical disc.
The structure and operation of dielectric resonators are described e.g. in the following articles:
[1] Ceramic Resonators for Highly Stable Oscillators, Gundolf Kuchler, Siemens Components XXIV (1989) No. 5, p. 180-183. PA1 [2] Microwave Dielectric Resonators, S. Jerry Fiedziuszko, Microwave Journal, September 1986, p. 189-191. PA1 [3] Cylindrical Dielectric Resonators and their Applications in TEM Line Microwave Circuits, Marian W. Pospieszalski, IEEE Transactions on Microwave Theory and Techniques, VOL. MTT-27, No. 3, March 1979, p. 233-238.
The resonance frequency of the dielectric resonator is primarily determined by the dimensions of the resonator element. Another factor affecting the resonance frequency is the surroundings of the resonator. The electric or magnetic field of the resonator and, thus, the resonance frequency can be intentionally affected by introducing a metal surface or any other conductive surface in the vicinity of the resonator. To adjust the resonance frequency of the dielectric resonator, a common practice is to adjust the distance between the conductive metal surface and the planar surface of the resonator. The adjusting mechanism may be e.g. an adjustment screw attached to the housing surrounding the resonator.
In this kind of adjusting method, however, it is typical that the resonance frequency varies nonlinearly as a function of the adjusting distance. Due to the non-linearity and the steepness of the adjustment, it is difficult and requires high precision to accurately adjust the resonance frequency, especially in the upper end of the adjusting range. In addition, the unloaded Q-factor varies as a function of the distance between the conductive surface and the resonator.
A constant Q-factor and a more linear frequency adjustment can be obtained within a wider range by replacing the conductive adjustment surface or plate with a dielectric adjustment plate wherein the distance from the planar surface of the resonator is adjusted. FIG. 7 in the above-mentioned article [2] shows a so-called double resonator structure as a modification of this solution. In the double resonator structure, two cylindrical dielectric resonator discs are positioned co-axially close to each other so that the distance between their planar surfaces can be adjusted by displacing the discs in the direction of their common axis. Also, in this case, the adjustment curve is still steep. In addition the double resonator structure is larger and more complicated than a conventional structure utilizing an adjustment plate.